There has been commercially available a type of force sensor in which an external force is exerted on a diaphragm portion which is made of a flexible thin plate, thereby electrically detecting a deformed state of the diaphragm portion and also detecting a magnitude of the exerted external force and a direction thereof. This type of sensor is relatively simple in structure and can be kept low in production cost, and can be used in many applications in various industries as a mass-produced product.
A method for electrically detecting a deformed state of the diaphragm portion includes a method for utilizing an electrostatic capacitive element, a method for utilizing a piezoresistive element and a method for utilizing a piezoelectric element. Each of these elements is disposed at a plurality of sites to detect displacement of each position at the diaphragm portion as an electrical signal, thus making it possible to detect independently a magnitude of a component of an exerted external force in the direction of each coordinate axis in an XYZ three-dimensional orthogonal coordinate system.
For example, in Patent Documents 1 and 2 given below, there is disclosed a force sensor which is capable of utilizing an electrostatic capacitive element to detect displacement of each site of the diaphragm portion, thereby detecting independently a force exerted in a direction of each coordinate axis. Further, in Patent Documents 3 and 4, there is disclosed a force sensor which detects a force in a direction of each coordinate axis on the basis of change in electric resistance of a piezoresistive element installed at individual sites of the diaphragm portion. In Patent Documents 5 and 6, there is disclosed a multi-axis sensor which detects displacement of each site of a diaphragm portion on the basis of electrical charge occurring at a piezoelectric element, detecting an acceleration-based force exerted as an external force and coriolis force, thereby detecting acceleration in a direction of each coordinate axis and angular speed around each coordinate axis.
The diaphragm portion of each of the above-described sensors functions as a flexibly deformed portion which causes elastic deformation and undergoes deformation upon exertion of an external force but returns to an original state when no external force is found. In general, in order to increase the detection sensitivity of force, the diaphragm portion is required to increase its flexibility by decreasing the thickness of the diaphragm portion or making the diaphragm portion with a material which can be easily deflected. However, exertion of an excessive external force will cause excessive deflection beyond the limit of elastic deformation at the diaphragm portion. This excessive deflection causes problems, for example, the diaphragm portion may not return to its original shape and the diaphragm portion may be broken after removal of the external force. In particular, in a force sensor with high sensitivity into which a sensitive diaphragm portion which causes deflection upon exertion of a very slight external force is assembled, the diaphragm portion may be more likely to be broken when an excessive external force is exerted. Therefore, in Patent Document 7, disclosed is such technology that a stopper member is used to control displacement so that a diaphragm portion undergoes displacement within a predetermined tolerance even upon exertion of an excessive external force.